The present invention relates to a superconducting wire or cable having a twisted filament structure utilized for a superconducting device operative with commercial frequency.
In a conventional technique, an A.C. superconducting wire is formed of an extremely fine multiconductor wire composed of tens of thousands of superconducting filaments each having a diameter less than 1 .mu.m. According to the realization of such technique, the electricity loss in the A.C. excitation has been reduced. However, the material of each filament is made extremely thin, so that one superconducting filament material has a current capacity only of about several tens of amperes (A).
In order to make large the current capacity, there has been proposed a superconducting wire having twisted filament structure, and in a certain case, a plurality of the thus twisted filament structures are further twisted to obtain further increased current capacity, such as shown in "Development of KA-class Superconducting Cables for AC use", by T. Hamajima, 11th Magnet Technology, 610 (1989).
Generally, the superconducting wire has an electric resistance which becomes zero at a very low temperature, and accordingly, when an electric current is supplied to a twisted filament type superconducting wire from an external power source, a mode of shunt electric current is determined in accordance with inductances of the respective superconducting material filaments constituting the superconducting wire or cable and a connection resistance between the power source and the respective superconducting material filaments.
However, in a case where an alternating current having a commercial frequency such as of 50 or 60 Hz, or more is conducted, the resistance caused by the inductance is mainly weighed and, hence, the shunt current is decided only by the inductance. For this reason, when an alternating current having the commercial frequency or more is conducted to a superconducting wire of conventional structure, a current is not flown uniformly through the respective superconducting material filaments due to the difference in the inductances between an inside superconducting material filament and outside ones in a bundle of filaments. This may be caused by the fact that the positions or arrangements of the respective material filaments are not changed even if the filaments are twisted. Namely, an inside filament is always positioned inside and an outside filament is always positioned outside.
In the meantime, when a plurality of the superconducting wires each of the structure described above are twisted, there causes a case where a current flows, in a converged manner, only in a certain specific superconducting wire, and in an extreme case where the currents exceeds the critical current value for the specific superconducting wire, a state of the superconducting wire will transfer to a normal-conducting state from the superconducting state. For the reason described, in the conventional technique, it is extremely diffcult to provide an A.C. superconducting wire or cable particularly adapted for large electric current capacity.